1. Field of Embodiments
The embodiments relate generally to a method and an installation for the intermittent production and continuous supply of a resin-filler mixture in the course of the production of plastic molded parts.
2. Description of Related Art
On account of their good mechanical properties and favorable prices, plastic molded parts and particularly molded parts produced from fiber-reinforced thermosetting plastics (SMC) are the most widely used thermosetting fiber-reinforced plastics in the plastics-processing industry. Until now, a type of raw material (semi-finished product) made of a resin compound (resin formulation) comprising embedded fillers and chopped or broken (or finitely long) glass fibers was initially manufactured for the production of SMC parts. The resin-filler mixture (resin formulation comprising fillers) is applied to thin plastic films (backing films), and the broken, chopped or finitely long fibers are deposited in the space between the plastic films.
After a fulling section used for an improved wetting of the fibers, the semi-finished product passes through a maturing process before it can be processed into the component. The usual maturing process takes three to five days and serves, inter alia, for thickening the mixture. The resin formulation itself usually consists of resins, thermoplastic solutions, additives for improving the material properties, integrated parting agents, inhibitors and optionally additional additives for modifying defined properties and a hardener.
The resin formulation is stirred at a predetermined temperature range to form a well-dispersed liquid, provided with mineral fillers, and supplemented at the SMC-production plant with a thickening agent in powder or liquid form, shortly before being applied to the films. The resin-filler mixture is then applied with doctor knives to the lower and upper backing films and adjusted in terms of thickness. The chopped glass, endless rovings, for example, can be applied in proper amounts to the resin-filler mixture layer of the lower backing film by means of chopping units. Thereafter, the upper backing film that is likewise provided with the resin-filler mixture is laid onto the chopped fiber coat and the lower layer. In a fulling process that follows, the fibers are mixed/saturated with the resin-filler mixture and then wound onto a roll as a sheet molding compound or stored intermediately as a flat semi-finished product for the maturing process. The maturing process usually makes the resin-filler mixture so strong that the backing films can be peeled off without leaving any residue. After or during the maturing period, blanks are cut from the flat semi-finished product or rolls to correspond to the molded part being produced, and they are placed and pressed singly or in layered packs in a molding press. The disadvantage of this method is that the SMC semi-finished product is produced in batch quantities and can be processed only after a maturing period lasting several hours or several days, as a result of which material inconsistencies and high storage and management costs are incorporated into the total production costs of a semi-finished product.
Another disadvantage is that the stored sheet molding compounds or the resin formulation wound on a roll can become distorted during the maturing process. If the roll lies on the floor, the bottom SMC layers are pressed and the material is thus displaced sideward. If the rolls are suspended on a shaft, the negative effect during maturation is the opposite: the material flows downward in the direction of the gravitational force and the web layers in the bottom part of the roll can have greater weights per unit area than the upper web layers. The unavoidable different weights per unit area, cause manufacturers to not only cut the sheet molding compounds lengthwise before the pressing operation, but also to weigh the sheet molding compounds to achieve equal bulk and thus equal part density for each part to be produced. If the weights per unit area of the webs were constant, the necessary SMC web pieces could be cut automatically with a length gauge, which would considerably reduce production costs and improve the quality of the molded parts. The composition of the cut blanks varying in thickness is also the reason why the production of SMC molded parts has never before been satisfactorily automated. Another disadvantage of the former SMC production is to be seen in the non-reusable backing films, which is another cost factor in the manufacturing process.
DE 102 33 300 A1 discloses a method and an apparatus that attempts to minimize the disadvantage of storage and use of film in that an encapsulated continuous thickening unit is disposed for the resin formulation after the production zone. The resin formulation is continuously guided between two endless belts and made to mature completely as it idles in the continuous thickening unit. The method and the apparatus are usable, in principle. However, the disadvantage of the prior art cited above is that it constantly necessitates an additional storage and maturation device that entails investment, operating and naturally maintenance costs that in turn increase the total production costs. Also, not all variants of SMC-formulations can be conveyed on the above apparatus since the admixture and mixing of all additives and reactants in the resin used repeatedly pose problems for the industry, which require expensive solutions and process engineering. In part, additives are admixed in proportions of 1:1000 (1 milliliter to 1 liter) or less. The ratio is even more extreme when additives have to be admixed after the admixture of fillers to the resin, and the problem of an absolutely uniform mixture also remains unresolved.
In principle, it can be noted that a generic resin is not used for the production of SMC components, but instead the resin production must correspond to the component to be produced in each case since the buyer can place other requirements depending on the component. Extreme conflicting examples in this case include plastic bumpers on a motor vehicle and an interior trim panel to be laminated such as the dashboard in a motor vehicle. Accordingly, it would be advantageous if the producer responsible could adjust and produce the material type of the plastic using simple means before the pressing operation, and need not laboriously produce or order the corresponding material days or weeks in advance. Furthermore, it is possible to detect errors in the formulations in the immediately subsequent component production and to readjust the installation engineering, whereas entire batches are present in the form of rejects in the case of matured systems based on semi-finished products.
DE 10 2006 062 622 A1 discloses a method and a device for the production of a resin mixture in the course of the production of plastic molded parts, particularly fiber-reinforced plastic-molded parts (SMC), in the uninterruptible direct process, the individual portions of a fluid main component being added and mixed for the production of the resin mixture. For this purpose, two mixing circuits are implemented each of which has a dynamic mixing zone and a subsequent static mixer. In a rotating circulation system, a dynamic mixing zone comprising one or more peristaltic pumps comprising injection nozzles for the circulation system, a pump, a static mixer and an overflow outlet that is intended for the premix and is made of a connection to the second mixing circuit are disposed in series in the first mixing circuit, and a dynamic mixing zone comprising one or more peristaltic pumps and/or gear pumps, a pump, a static mixer and an overflow outlet comprising a connection for the transfer of the resin mixture to the extruder are disposed in series in the second mixing circuit, the extruder comprising a supply inlet for fillers and reinforcing fibers. The solution suggested here has basically proved useful, but leaves scope for improvement.
In principle, the disadvantage of all methods and devices used in the mixing of formulation components, particularly in SMC production, by means of pumps is that highly viscous liquids and also very small quantities cannot be metered in accordance with the specifications. Critical metering differences can result in different properties in the further processing in the extruder or in a press or reduce the shelf life until further processing and thus lead to unusable material. Furthermore, at least one portion, if not more, of the liquids to be metered is fire-prone or explosion-prone during the heating process and requires high safety-related and accident prevention expenditure. In addition, volatile portions with features rendering them disadvantageous for handling are also to be found.
The additional problem confronted in rotating circuit mixers is that components are continuously supplied and resin mixture is continuously discharged due to the characteristics of the system, and reactive constituents can stay for a long duration in the mixing zone and lose their reactivity or they can lead to signs of partial cross-linking. This effect proves to be particularly disadvantageous when making alterations in the formulation, as a result of which it is not suitable for an online alteration of the formulation. Furthermore, plant components such as gear pumps, overflow valves and flow-measuring systems have proved to be very accident-sensitive and high-maintenance when handling highly viscous and reactive resins.